The surgical removal of a herniated disc, whether from degenerative disease or traumatic disruption, is a common procedure in current medical practice. In the cervical spine, the procedure involves placement of a large temporary bone screw, which is also known as the distraction screw, into each of the vertebral bones above and below the diseased disc space. These screws are used to realign the vertebral bones into the desired anatomical relationship and to temporarily distract them so as to permit work within the intervening disc space. The disc is removed and a bone graft or suitable graft substitute is placed into the evacuated space. The temporary distraction screws are then removed from the vertebrae and a metallic skeletal plate is used to maintain the position of the vertebral bones while bone healing occurs. The bones are fixed to the skeletal plate using implantable bone screws (usually two screws per vertebrae), which are separate and distinct from the distraction screws.
Removal of the distraction screws from the vertebral bodies usually produces robust bone bleeding and requires that the bone holes be filled with a hemostatic agent. The empty bone holes also act as stress concentration points within the vertebral bodies, as would any empty opening or crack within a rigid structural member, and predispose the vertebral bodies to bone fracture, screw/plate migration and construct failure. Further, the empty holes often interfere with proper placement of the implantable screws and the associated skeletal plate, making proper alignment of the plate along the anatomically desired plane more difficult. This is especially problematic since the plate is placed at the end of the operative procedure and the preceding surgical steps have distorted the anatomical landmarks required to ensure proper plate alignment.
Lastly, once placed, the plate will effectively cover the vertebral bodies of the reconstructed segment. Extension of the operation to an adjacent level at a future date will require placement of a distraction screw within a covered vertebrae and, thus, necessitate plate removal. The latter requires re-dissection through the scarred operative field of the initial procedure and significantly increases the operative risk of the second procedure for the patient.
In view of the above, it would be desirable to design an improved distraction screw. The new device should minimize blood loss, reduce the potential for stress concentration, maximize the likelihood of proper plate alignment, provide an additional point of fixation for the skeletal plate and provide a ready mechanism for distraction screw replacement at the time of surgical revision without obligatory plate removal.